Publications by Michael Johnston

Optical Description of Mesostructured Organic-Inorganic Halide Perovskite Solar Cells.

The journal of physical chemistry letters 6 (2015) 48-53

M Anaya, G Lozano, ME Calvo, W Zhang, MB Johnston, HJ Snaith, H Míguez

Herein we describe both theoretically and experimentally the optical response of solution-processed organic-inorganic halide perovskite solar cells based on mesostructured scaffolds. We develop a rigorous theoretical model using a method based on the propagation of waves in layered media, which allows visualizing the way in which light is spatially distributed across the device and serves to quantify the fraction of light absorbed by each medium comprising the cell. The discrimination between productive and parasitic absorption yields an accurate determination of the internal quantum efficiency. State-of-the-art devices integrating mesoporous scaffolds infiltrated with perovskite are manufactured and characterized to support the calculations. This combined experimental and theoretical analysis provides a rational understanding of the optical behavior of perovskite cells and can be beneficial for the judicious design of devices with improved performance. Notably, our model justifies the presence of a solid perovskite capping layer in all of the highest efficiency perovskite solar cells based on thinner mesoporous scaffolds.

Efficient, semitransparent neutral-colored solar cells based on microstructured formamidinium lead trihalide perovskite.

Journal of Physical Chemistry Letters American Chemical Society 6 (2015) 129-138

GE Eperon, D Bryant, J Troughton, SD Stranks, M Johnston, T Watson, DA Worsley, HJ Snaith

Efficient, neutral-colored semitransparent solar cells are of commercial interest for incorporation into the windows and surfaces of buildings and automobiles. Here, we report on semitransparent perovskite solar cells that are both efficient and neutral-colored, even in full working devices. Using the microstructured architecture previously developed, we achieve higher efficiencies by replacing methylammonium lead iodide perovskite with formamidinium lead iodide. Current-voltage hysteresis is also much reduced. Furthermore, we apply a novel transparent cathode to the devices, enabling us to fabricate neutral-colored semitransparent full solar cells for the first time. Such devices demonstrate over 5% power conversion efficiency for average visible transparencies of almost 30%, retaining impressive color-neutrality. This makes these devices the best-performing single-junction neutral-colored semitransparent solar cells to date. These microstructured perovskite solar cells are shown to have a significant advantage over silicon solar cells in terms of performance at high incident angles of sunlight, making them ideal for building integration.

Colour-selective photodiodes

NATURE PHOTONICS 9 (2015) 633-636

MB Johnston

Single nanowire photoconductive terahertz detectors.

Nano letters 15 (2015) 206-210

K Peng, P Parkinson, L Fu, Q Gao, N Jiang, Y-N Guo, F Wang, HJ Joyce, JL Boland, HH Tan, C Jagadish, MB Johnston

Spectroscopy and imaging in the terahertz (THz) region of the electromagnetic spectrum has proven to provide important insights in fields as diverse as chemical analysis, materials characterization, security screening, and nondestructive testing. However, compact optoelectronics suited to the most powerful terahertz technique, time-domain spectroscopy, are lacking. Here, we implement single GaAs nanowires as microscopic coherent THz sensors and for the first time incorporated them into the pulsed time-domain technique. We also demonstrate the functionality of the single nanowire THz detector as a spectrometer by using it to measure the transmission spectrum of a 290 GHz low pass filter. Thus, nanowires are shown to be well suited for THz device applications and hold particular promise as near-field THz sensors.

Optical properties and limiting photocurrent of thin-film perovskite solar cells

Energy and Environmental Science Royal Society of Chemistry 8 (2014) 602–609-

JM Ball, SD Stranks, MT Hörantner, S Hüttner, W Zhang, EJW Crossland, I Ramirez, M Riede, M Johnston, RH Friend, H Snaith

Metal-halide perovskite light-absorbers have risen to the forefront of photovoltaics research offering the potential to combine low-cost fabrication with high power-conversion efficiency. Much of the development has been driven by empirical optimisation strategies to fully exploit the favourable electronic properties of the absorber layer. To build on this progress, a full understanding of the device operation requires a thorough optical analysis of the device stack, providing a platform for maximising the power conversion efficiency through a precise determination of parasitic losses caused by coherence and absorption in the non-photoactive layers. Here we use an optical model based on the transfer-matrix formalism for analysis of perovskite-based planar heterojunction solar cells using experimentally determined complex refractive index data. We compare the modelled properties to experimentally determined data, and obtain good agreement, revealing that the internal quantum efficiency in the solar cells approaches 100%. The modelled and experimental dependence of the photocurrent on incidence angle exhibits only a weak variation, with very low reflectivity losses at all angles, highlighting the potential for useful power generation over a full daylight cycle.

Single nanowire terahertz detectors

CLEO: Science and Innovations, CLEO-SI 2015 (2015) 2267-

K Peng, P Parkinson, L Fu, Q Gao, N Jiang, YN Guo, F Wang, HJ Joyce, JL Boland, MB Johnston, HH Tan, C Jagadish

Photoconductive terahertz detectors based on single GaAs/AlGaAs nanowire were designed, fabricated and incorporated into the pulsed time-domain technique, showing a promise for nanowires in terahertz applications such as near-field terahertz sensors or on-chip terahertz micro-spectrometers. © OSA 2015.

Plasmonic-Induced Photon Recycling in Metal Halide Perovskite Solar Cells

Advanced Functional Materials 25 (2015) 5038-5046

M Saliba, W Zhang, VM Burlakov, SD Stranks, Y Sun, JM Ball, MB Johnston, A Goriely, U Wiesner, HJ Snaith

© 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim. Organic-inorganic metal halide perovskite solar cells have emerged in the past few years to promise highly efficient photovoltaic devices at low costs. Here, temperature-sensitive core-shell Ag@TiO<inf>2</inf> nanoparticles are successfully incorporated into perovskite solar cells through a lowerature processing route, boosting the measured device efficiencies up to 16.3%. Experimental evidence is shown and a theoretical model is developed which predicts that the presence of highly polarizable nanoparticles enhances the radiative decay of excitons and increases the reabsorption of emitted radiation, representing a novel photon recycling scheme. The work elucidates the complicated subtle interactions between light and matter in plasmonic photovoltaic composites. Photonic and plasmonic schemes such as this may help to move highly efficient perovskite solar cells closer to the theoretical limiting efficiencies.

FTU Diagnostic System based on THz Time-Domain Spectroscopy


F Causa, M Zerbini, M Johnston, O Tudisco, P Buratti, A Doria, GP Gallerano, E Giovenale, AA Tuccillo

Modulation doping of GaAs/AlGaAs core-shell nanowires with effective defect passivation and high electron mobility

Nano letters American Chemical Society 15 (2015) 1336-1342

JL Boland, S Conesa-Boj, P Parkinson, G Tütüncüoglu, F Matteini, D Rüffer, A Casadei, F Amaduzzi, F Jabeen, CL Davies, H Joyce, L Herz, A Fontcuberta i Morral, M Johnston

Reliable doping is required to realize many devices based on semiconductor nanowires. Group III-V nanowires show great promise as elements of high-speed optoelectronic devices, but for such applications it is important that the electron mobility is not compromised by the inclusion of dopants. Here we show that GaAs nanowires can be n-type doped with negligible loss of electron mobility. Molecular beam epitaxy was used to fabricate modulation-doped GaAs nanowires with Al0.33Ga0.67As shells that contained a layer of Si dopants. We identify the presence of the doped layer from a high-angle annular dark field scanning electron microscopy cross-section image. The doping density, carrier mobility, and charge carrier lifetimes of these n-type nanowires and nominally undoped reference samples were determined using the noncontact method of optical pump terahertz probe spectroscopy. An n-type extrinsic carrier concentration of 1.10 ± 0.06 × 10(16) cm(-3) was extracted, demonstrating the effectiveness of modulation doping in GaAs nanowires. The room-temperature electron mobility was also found to be high at 2200 ± 300 cm(2) V(-1) s(-1) and importantly minimal degradation was observed compared with undoped reference nanowires at similar electron densities. In addition, modulation doping significantly enhanced the room-temperature photoconductivity and photoluminescence lifetimes to 3.9 ± 0.3 and 2.4 ± 0.1 ns respectively, revealing that modulation doping can passivate interfacial trap states.

Plasmonic-induced photon recycling in metal halide perovskite solar cells

Advanced Functional Materials Wiley 25 (2015) 5038-5046

M Saliba, W Zhang, V Burlakov, M Johnston, A Goriely, H Snaith, E al.

Organic–inorganic metal halide perovskite solar cells have emerged in the past few years to promise highly effi cient photovoltaic devices at low costs. Here, temperature-sensitive core–shell Ag@TiO 2 nanoparticles are successfully incorporated into perovskite solar cells through a low-temperature processing route, boosting the measured device efficiencies up to 16.3%. Experimental evidence is shown and a theoretical model is developed which predicts that the presence of highly polarizable nanoparticles enhances the radiative decay of excitons and increases the reabsorption of emitted radiation, representing a novel photon recycling scheme. The work elucidates the complicated subtle interactions between light and matter in plasmonic photovoltaic composites. Photonic and plasmonic schemes such as this may help to move highly efficient perovskite solar cells closer to the theoretical limiting efficiencies.

Fast charge-carrier trapping in TiO2 nanotubes

Journal of Physical Chemistry C American Chemical Society 119 (2015) 9159-9168

C Wehrenfennig, CM Palumbiny, HJ Snaith, M Johnston, L Schmidt-Mende, L Herz

One-dimensional semiconductors such as nanowires and nanotubes are attractive materials for incorporation in photovoltaic devices as they potentially offer short percolation pathways to charge-collecting contacts. We report the observation of free-electron lifetimes in TiO<inf>2</inf> nanotubes of the order of tens of picoseconds. These lifetimes are surprisingly short compared to those determined in films of TiO<inf>2</inf> nanoparticles. Samples of ordered nanotube arrays with several different tube wall thicknesses were fabricated by anodization and have been investigated by means of optical-pump-terahertz-probe (OPTP) spectroscopy, which allows measurement of transient photoinduced conductivity with picosecond resolution. Our results indicate a two-stage decay of the photoexcited electron population. We attribute the faster component to temporary immobilization of charge in shallow trap states, from which electrons can detrap again by thermal excitation. The slower component most likely reflects irreversible trapping in states deeper below the conduction band edge. Free-electron lifetimes associated with shallow trapping appear to be independent of the tube wall thickness and have very similar values for electrons directly photoexcited in the material and for those injected from an attached photoexcited dye. These results suggest that trap states are not predominantly located at the surface of the tubes. The effective THz charge-carrier mobility in the TiO<inf>2</inf> nanotubes is determined (0.1-0.4 cm<sup>2</sup>/(Vs)) and found to be within the same range as carrier mobilities reported for TiO<inf>2</inf> nanoparticles. Implications for the relative performance of these nanostructures in dye-sensitized solar cells are discussed.

In(x)Ga(1-x)As nanowires with uniform composition, pure wurtzite crystal phase and taper-free morphology.

Nanotechnology 26 (2015) 205604-

AS Ameruddin, HA Fonseka, P Caroff, J Wong-Leung, RLM Op het Veld, JL Boland, MB Johnston, HH Tan, C Jagadish

Obtaining compositional homogeneity without compromising morphological or structural quality is one of the biggest challenges in growing ternary alloy compound semiconductor nanowires. Here we report growth of Au-seeded InxGa1-xAs nanowires via metal-organic vapour phase epitaxy with uniform composition, morphology and pure wurtzite (WZ) crystal phase by carefully optimizing growth temperature and V/III ratio. We find that high growth temperatures allow the InxGa1-xAs composition to be more uniform by suppressing the formation of typically observed spontaneous In-rich shells. A low V/III ratio results in the growth of pure WZ phase InxGa1-xAs nanowires with uniform composition and morphology while a high V/III ratio allows pure zinc-blende (ZB) phase to form. Ga incorporation is found to be dependent on the crystal phase favouring higher Ga concentration in ZB phase compared to the WZ phase. Tapering is also found to be more prominent in defective nanowires hence it is critical to maintain the highest crystal structure purity in order to minimize tapering and inhomogeneity. The InP capped pure WZ In0.65Ga0.35As core-shell nanowire heterostructures show 1.54 μm photoluminescence, close to the technologically important optical fibre telecommunication wavelength, which is promising for application in photodetectors and nanoscale lasers.

Terahertz Spectroscopy of Modulation Doped Core-Shell GaAs/AlGaAs Nanowires


JL Boland, S Conesa-Boj, G Tutuncouglu, F Matteini, D Ruffer, A Casadei, F Gaveen, F Amaduzzi, P Parkinson, C Davies, HJ Joyce, LM Herz, A Fontcuberta i Morral, MB Johnston, IEEE

Photoconductive terahertz receivers utilizing single semiconductor nanowires


K Peng, P Parkinson, L Fu, Q Gao, N Jiang, Y-N Guo, F Wang, HJ Joyce, JL Boland, HH Tan, C Jagadish, MB Johnston, IEEE

Charge-Carrier Dynamics and Mobilities in Formamidinium Lead Mixed-Halide Perovskites

Advanced Materials Wiley (2015) n/a-n/a

W Rehman, RL Milot, GE Eperon, C Wehrenfennig, JL Boland, HJ Snaith, MB Johnston, L Herz

The mixed-halide perovskite FAPb(BryI1–y)3 is attractive for color-tunable and tandem solar cells. Bimolecular and Auger charge-carrier recombination rate constants strongly correlate with the Br content, y, suggesting a link with electronic structure. FAPbBr3 and FAPbI3 exhibit charge-carrier mobilities of 14 and 27 cm2 V−1 s−1 and diffusion lengths exceeding 1 μm, while mobilities across the mixed Br/I system depend on crystalline phase disorder.

Enhanced Amplified Spontaneous Emission in Perovskites using a Flexible Cholesteric Liquid Crystal Reflector

Nano letters American Chemical Society 15 (2015) 4935-4941

SD Stranks, SM Wood, K Wojciechowski, F Deschler, M Saliba, H Khandelwal, JB Patel, SJ Elston, L Herz, M Johnston, APHJ Schenning, MG Debije, M Riede, SM Morris, HJ Snaith

Organic-inorganic perovskites are highly promising solar cell materials with laboratory-based power conversion efficiencies already matching those of established thin film technologies. Their exceptional photovoltaic performance is in part attributed to the presence of efficient radiative recombination pathways, thereby opening up the possibility of efficient light-emitting devices. Here, we demonstrate optically pumped amplified spontaneous emission (ASE) at 780 nm from a 50 nm-thick film of CH3NH3PbI3 perovskite that is sandwiched within a cavity composed of a thin-film (∼7 μm) cholesteric liquid crystal (CLC) reflector and a metal back-reflector. The threshold fluence for ASE in the perovskite film is reduced by at least two orders of magnitude in the presence of the CLC reflector, which results in a factor of two reduction in threshold fluence compared to previous reports. We consider this to be due to improved coupling of the oblique and out-of-plane modes that are reflected into the bulk in addition to any contributions from cavity modes. Furthermore, we also demonstrate enhanced ASE on flexible reflectors and discuss how improvements in the quality factor and reflectivity of the CLC layers could lead to single-mode lasing using CLC reflectors. Our work opens up the possibility of fabricating widely wavelength-tunable "mirror-less" single-mode lasers on flexible substrates, which could find use in applications such as flexible displays and friend or foe identification.

Highly efficient perovskite solar cells with tunable structural color

Nano Letters American Chemical Society 15 (2015) 1698-1702

W Zhang, M Anaya, G Lozano, ME Calvo, M Johnston, H Míguez, H Snaith

The performance of perovskite solar cells has been progressing over the past few years and efficiency is likely to continue to increase. However, a negative aspect for the integration of perovskite solar cells in the built environment is that the color gamut available in these materials is very limited and does not cover the green-to-blue region of the visible spectrum, which has been a big selling point for organic photovoltaics. Here, we integrate a porous photonic crystal (PC) scaffold within the photoactive layer of an opaque perovskite solar cell following a bottom-up approach employing inexpensive and scalable liquid processing techniques. The photovoltaic devices presented herein show high efficiency with tunable color across the visible spectrum. This now imbues the perovskite solar cells with highly desirable properties for cladding in the built environment and encourages design of sustainable colorful buildings and iridescent electric vehicles as future power generation sources.

Charge selective contacts, mobile ions and anomalous hysteresis in organic-inorganic perovskite solar cells

Materials Horizons Royal Society of Chemistry 2 (2015) 315-322

Y Zhang, M Liu, G Eperon, TC Leijtens, D McMeekin, M Saliba, W Zhang, M de Bastiani, A Petrozza, L Herz, M Johnston, H Lin, HJ Snaith

High-efficiency perovskite solar cells typically employ an organic–inorganic metal halide perovskite material as light absorber and charge transporter, sandwiched between a p-type electron-blocking organic hole-transporting layer and an n-type hole-blocking electron collection titania compact layer. Some device configurations also include a thin mesoporous layer of TiO2 or Al2O3 which is infiltrated and capped with the perovskite absorber. Herein, we demonstrate that it is possible to fabricate planar and mesoporous perovskite solar cells devoid of an electron selective hole-blocking titania compact layer, which momentarily exhibit power conversion efficiencies (PCEs) of over 13%. This performance is however not sustained and is related to the previously observed anomalous hysteresis in perovskite solar cells. The “compact layer-free” meso-superstructured perovskite devices yield a stabilised PCE of only 2.7% while the compact layer-free planar heterojunction devices display no measurable steady state power output when devoid of an electron selective contact. In contrast, devices including the titania compact layer exhibit stabilised efficiency close to that derived from the current voltage measurements. We propose that under forward bias the perovskite diode becomes polarised, providing a beneficial field, allowing accumulation of positive and negative space charge near the contacts, which enables more efficient charge extraction. This provides the required built-in potential and selective charge extraction at each contact to temporarily enable efficient operation of the perovskite solar cells even in the absence of charge selective n- and p-type contact layers. The polarisation of the material is consistent with long range migration and accumulation of ionic species within the perovskite to the regions near the contacts. When the external field is reduced under working conditions, the ions can slowly diffuse away from the contacts redistributing throughout the film, reducing the field asymmetry and the effectiveness of the operation of the solar cells. We note that in light of recent publications showing high efficiency in devices devoid of charge selective contacts, this work reaffirms the absolute necessity to measure and report the stabilised power output under load when characterizing perovskite solar cells.

Ultrafast transient terahertz conductivity of monolayer MoS₂ and WSe₂ grown by chemical vapor deposition

ACS nano American Chemical Society 8 (2014) 11147-11153

CJ Docherty, P Parkinson, H Joyce, M-H Chiu, C-H Chen, M-Y Lee, L-J Li, L Herz, M Johnston

We have measured ultrafast charge carrier dynamics in monolayers and trilayers of the transition metal dichalcogenides MoS2 and WSe2 using a combination of time-resolved photoluminescence and terahertz spectroscopy. We recorded a photoconductivity and photoluminescence response time of just 350 fs from CVD-grown monolayer MoS2, and 1 ps from trilayer MoS2 and monolayer WSe2. Our results indicate the potential of these materials as high-speed optoelectronic materials.

Solution Deposition-Conversion for Planar Heterojunction Mixed Halide Perovskite Solar Cells


P Docampo, FC Hanusch, SD Stranks, M Doeblinger, JM Feckl, M Ehrensperger, NK Minar, MB Johnston, HJ Snaith, T Bein